1. Field of the Invention
The present invention relates to an improved method for fabricating a fiber-reinforced ceramic matrix composite, and more particularly, relates to a method for fabricating an article in a short period by impregnating a matrix substance.
2. Background Art
In order to increase performance of a rocket engine consuming, as fuel, NTO/N2H4, NTO/MMH, and the like, it is necessary that the combustion chamber (thrust producing chamber) of the rocket engine have extremely high thermal resistance. A niobium alloy, with a coating, able to resist temperatures of up to about 1,500xc2x0 C. has been used for forming combustion chambers of numerous rocket engines so as to meet the thermal requirement. On the other hand, drawbacks of this material are that it is heavy due to its high density and that it exhibits low strength at high temperature, and in addition, the service life of its coating is relatively short.
Although ceramics exhibit excellent thermal resistance, they are brittle. In order to overcome the defect, a ceramic matrix composite reinforced with ceramic fibers (hereinafter abbreviated as CMC) has been developed. A CMC consists of ceramic fibers and a ceramic matrix. A CMC is generally specified in the form of ceramic fibers/ceramic matrix. For example, if a CMC consists of a ceramic fiber of silicon carbide (hereinafter abbreviated as SiC) and a ceramic matrix of SiC, this CMC is specified as SiC/SiC.
Because CMC is relatively light and has superior thermal resistance, it may be preferably used not only in combustion chambers (thrust producing chambers) in rocket engines described above, but also in fuel pipes which will be exposed to high temperatures, turbine blades in jet engines, combustion chambers thereof, and afterburner parts thereof.
On the other hand, CMCs tend to fail to be airtight and to exhibit low thermal shock resistance. When a CMC is used as a material for an article, first, the shape of the article is created with ceramic fibers, and then a matrix is formed in the voids among the fibers by chemical vapor infiltration (hereinafter abbreviated as CVI treatment). It takes a long time, e.g., more than a year, which is impractical, to completely fill the voids by a CVI treatment, which is a serious problem. If the article made of an ordinary CMC by the above method is tested at a high temperature and is exposed to a severe thermal shock, e.g., a temperature difference of more than 900xc2x0 C., the strength of the article is drastically reduced, and consequently, the article cannot be used. Therefore, it has been generally believed that an ordinary CMC is not suitable for elements such as combustion chambers (thrust producing chambers) and fuel pipes which must provide both airtight performance and thermal shock resistance.
Japanese Unexamined Patent Application, First Publication, No. 2000-219576 discloses a fabrication method for overcoming the above problem. This method uses a treatment consisting of polymer impregnation and pyrolysis (hereinafter abbreviated as PIP treatment). The method comprises the steps of shaping a woven fabric into the desired shape of an article; forming a SiC matrix layer on the surface of the woven fabric by a CVI treatment; impregnating an organic silicon polymer, as a base material, into voids in the matrix layer, and pyrolyzing the organic silicon polymer by a PIP treatment. Because a matrix is formed more quickly by a PIP treatment than by a CVI treatment, and because a PIP treatment can be repeatedly performed in a short period, the remaining voids in the matrix layer after a CVI treatment can be completely filled by repeated PIP treatments, whereby airtightness of the article is improved. Since the matrix formed by PIP treatments has microcracks and the binding force among the ceramic fibers is relatively low, the Young""s modulus of the CMC is reduced when a PIP treatment is performed in addition to a CVI treatment than when only a CVI treatment is applied for forming the CMC as was usual. As a result, thermal stress is reduced and thermal shock resistance can be much improved.
In such known processes for fabricating an article with CMC, the shape of the article is formed by machining after a sufficient density of the matrix layer is achieved by a PIP treatment, and then finally another coating process is performed by a CVI treatment. For example, in the case of fabricating a chamber of a rocket engine and the like as the article, the chamber must have a high degree of airtight performance, i.e., must achieve so-called no-leak performance upon completion in order to prevent combustion gas from escaping. In order to meet the no-leak requirement, a leak test is performed after coating by a CVI treatment, and depending on the leak test result, further PIP treatments and coating processes by CVI treatments may be repeated as necessary.
The conventional method for fabricating a CMC above described, however, includes some drawbacks as follows.
Since the voids in the woven fabric are filled or covered by the matrix layer formed near the outer surface of the woven fabric through the CVI treatment, a polymer cannot be easily impregnated into inside the woven fabric during the PIP treatment. Because of this, the PIP treatments must be performed up to 30 to 40 times in order to achieve a desired density of the polymer by filling the polymer, and thus to achieve no-leak performance, which means that throughput is very low.
Based on the above problem, an object of the present invention is to provide a method for fabricating a CMC article by which throughput of the CMC articles is increased.
In order to achieve the above object, the method according to the present invention provides the following.
A method, according to a first aspect of the present invention, for fabricating a CMC article, comprises performing a CVI treatment for forming a SiC matrix layer on the surface of a woven fabric; performing a machining process, after the CVI treatment, for machining the woven fabric; and performing a PIP treatment, after said machining process, for impregnating an organic silicon polymer as a base material into voids in the matrix layer, and pyrolyzing the organic silicon polymer.
In the above method, the matrix layer, which is formed in an outer portion of the woven fabric by the CVI treatment and covers the voids, can be removed by the machining process. As a result, impregnation paths are preferably provided in the PIP treatment, through which an organic silicon polymer can be easily impregnated into the inside portion of the woven fabric; thus, the number of the repeated PIP treatments may be reduced.
In a method according to a second aspect of the present invention for fabricating a CMC article, in addition to the processes in the first aspect, the woven fabric is machine-finished into a desired shape in the machining process.
In the above method, because the matrix layer, which is formed in outer portion of the woven fabric and covers the voids, can be removed and the woven fabric is machine-finished into a desired shape, another finishing process after the PIP treatment is not required.
A method, according to a third aspect of the present invention, for fabricating a CMC article, in addition to the processes in the first or second aspect, further comprises performing a slurry impregnating process before or after the PIP treatment for impregnating SiC in a slurry state into voids in the matrix layer.
In the above method, the filling factor can be efficiently increased, and consequently, the number of the repeated PIP treatments may be further reduced.